Water-blocked optical cable and process for the production thereof

ABSTRACT

An optical cable for communication includes at least one retaining element blocked with respect to the water propagation as well as a process for manufacturing such an optical cable. The optical cable includes, in addition to the retaining element, at least two transmission elements housed within the retaining element and a water swellable yarn housed within the retaining element. The water swellable yarn is selected according to the following equation: 
                       V   w       V   TF       =       k     V   t       +   R             (   1   )               
in which V w  is the volume of the water swellable yarn after swelling upon contact with water; V TF  is the total free volume in the retaining element; k is a constant ≧180; R is a constant ≧1.4; and V t  is the free volume per each transmission element. Advantageously, the optical cable is water-blocked and the water swellable yarn does not induce microbending effects on the transmission elements.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/EP2004/012631, filed Nov. 5, 2004, the content of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an optical cable comprising at leastone retaining element blocked with respect to the water propagation.

The present invention also relates to a process for manufacturing suchan optical cable.

STATE OF THE ART

In an optical cable, the transmission elements are, typically, opticalfibers. The optical fibers generally comprise a silica glass“core+cladding” transmitting element and an outer single or compositepolymeric layer (protecting coating) advantageously including a coloredlayer for identification.

The optical cable typically comprises buffering elements in radiallyexternal position with respect to the optical fibers, providingfunctions such as mechanical isolation, protection from physical damageand fiber identification.

For instance, one or more optical fibers, e.g. arranged in group, bundleor ribbon, can be housed in a tube or flexible sheath (hereinafterreferred to as “retaining element”) of polymeric material endowed withspecific mechanical properties (such as Young modulus, tensile strengthand elongation at break) in order to ensure an adequate protection tothe fibers.

The optical fiber/retaining element assembly is generally referred to as“optic unit”.

Among the optical cables in which the optical fiber(s) are housed in atubular retaining element, there are cables in which the opticalfiber(s) are inserted in a tube, sometimes called “buffer tube” or“loose tube”, providing fiber protection and identification. Theretaining element of this kind of optic unit usually has a thicknesshigher than about 0.2 mm, typically of from about 0.3 mm to about 0.8mm, and an inner diameter of 1.6-1.8 mm, when contains twelve opticalfibers.

In a specific type of tubular type optical cable, the optical units havereduced dimensions both in term of diameter and sheath thickness.Typically, the optical units are called “micromodules”, and theretaining element thereof is generally referred to as “microsheath” or“minisheath”. In this case, the retaining element material isparticularly designed for allowing the identification of fibers or ofgroups of fibers, and for achieving an easy access to the opticalfibers, e.g. by simply tearing and slipping off the retaining element,in order to facilitate both the connection between the optical fibersand the system equipment or the interconnection between cables. Themicrosheath is typically made of a material having a relatively lowmodulus of elasticity and ultimate elongation, such as PVC, ethyl-vinylacetate (EVA) and polyethylene. Advantageously, the use of the abovematerials for forming a thin microsheath also results in a microsheaththat is easier to remove or to strip, just using fingers or fingernails.In a typical micromodule optical cable, a retaining element containingtwelve optical fibers has an inner diameter of about 1.1 mm, and athickness of 0.2 mm or less, for example 0.15 mm.

A micromodule optical cable is known, for instance, from WO00/58768 (inthe Applicant's name), and comprises a number of micromodules, an innertube surrounding the micromodules and an external sheath covering theinner tube. The micromodules can optionally and advantageously showdifferent colors to be distinguished one another.

U.S. Pat. No. 5,155,789 (in the name of Société Industrielle de LiaisonsÉlectriques SILEC and État Français (Centre National d'Étude desTélécommunications—CNET) provides a telecommunication cable comprisingoptical fibers split into modules, each module being enveloped in a thinsupporting sheath that is easily torn, wherein the supporting sheathsare in contact with the optical fibers to clamp them together.

The arrangement of the optical fibers in micromodules as defined aboveallows employing a high number of optical fibers in a relatively smalloptical cable. The micromodule arrangement can provide, e.g., up to 144optical fibers in an optical cable having a diameter lower than or equalto 13 mm (this diameter is not comprehensive of additional protectinglayers optionally provided for specific purposes and requirements),making such a cable particularly suitable for urban distributionnetworks.

In the present description and claims, “blocked with respect to thewater propagation” means that the water propagation, mainly intended asa spreading along the longitudinal direction of the micromodule as aconsequence of a damage to the cable integrity, which results in aprogressive filling thereof, is substantially prevented or limited. Boththe micromodule and the cable containing it should fulfill therequirements of the test according to method F5B provided byinternational standard IEC 60794-1-2: further details regarding thistest will be provided hereinafter.

Typically, each micromodule can comprise from 2 to 12 optical fibershoused in a retaining element as from above.

The intrusion of water or humidity into an optical cable or amicromodule, and the consequent propagation therethrough can be aproblem. Water entering in the micromodule can migrate through itimpairing the transmission properties of the optical fibers housedtherein. Also, water can reach and degrade closure or other terminationdevice and/or can damage electronics mounted within the closure or othertermination device.

Methods are known for preventing such propagation. For example,micromodules and cables comprising the same are known which arewater-blocked by means of filling material included in differentpassageways. More particularly, a filler material can be included in theretaining element of each micromodule among the optical fibers containedtherein.

US 2003/0168243 (Jamet et al.) relates to a telecommunication cableincluding a plurality of modules which each have a thin retaining sheathclamping optical fibers together, and a jacket around the modules ischaracterized in that it comprises retaining sheaths which each containa plurality of respective modules and each of which is mechanicallycoupled to the retaining sheaths of the respective modules to formsupermodules in contact with the jacket.

A filler material, e.g. a sealing product such as silicone or syntheticgrease, oil or gel, or a “dry” product obtained by associating swellingpowder and/or swelling filaments and/or swelling tapes that swell in thepresence of water to form a stopper that prevents water propagation canbe provided inside the micromodule.

As pointed out, for example, by U.S. Pat. No. 5,157,752 (in the name ofNorthern Telecom Ltd.), there are problems associated with the use ofgreases or gels. For instance, such materials are difficult and costlyto apply into and fill cable passageways. Grease or gel also makes itdifficult and unpleasant to handle the fibers during installation orrepair of a cable, and at low temperatures (e.g. below 0° C.) change inviscosity of the grease or gel surrounding and contacting fibers mayincrease signal attenuation in the fibers. A further problem is thatsince greases or gels may be incompatible with economically desirableplastics, which could normally be extruded as tubes for containing thefibers, more expensively engineered polymers may be required for thetubes.

The use of a “dry” product could circumvent the problems associated withgel and grease.

The above-mentioned U.S. Pat. No. 5,157,752 discloses an optical cabledefining an axially extending passageway and an optical fiber means anda water blocking means disposed within and extending along thepassageway, the water blocking means comprising an elongate elementwhich swells upon contact with water to block the passageway against theflow of water.

The Applicant observed that the choice of the “dry” water swellablematerial, e.g. a water swellable yarn or a water swellable powderoptionally supported on an elongated carrier, for obtaining amicromodule blocked with respect to the water propagation, is a problem.

The water swellable material has to coexist with the optical fiberswithout causing damages thereto. For example, stresses arising from thecontact with the water swellable material can induce microbending in theoptical fibers and impair the transmission performance thereof.

In particular, the Applicant observed that commercially available waterswellable powders, dispersed among optical fibers or supported, e.g., bya filament provided inside the retaining element, may efficientlyprevent the water propagation along the micromodule, but, due to theirgrain size, typically of micrometer order of magnitude or larger, canimpair the optical fiber transmission properties by microbending. Thepowder can also yield agglomerates exacerbating the microbendingphenomenon.

The Applicant also observed that the grinding of said powders todecrease their grain size spoils their swelling capability. On the otherside, powders with grain size in the nanometer order, either obtained bygrinding or by processes other than grinding, give rise to problemsconnected with cost and handling during the cable production, and withhealth for the operators.

In addition, Applicant observed that a uniform and controlled physicaldistribution of such powders inside the micromodule is difficult to beobtained from the industrial point of view.

Water swellable yarns have been considered as an alternative to thewater swellable powder.

In the present description and claims, as “water swellable yarn” it isintended a water swellable tape or filament optionally supported by orstranded with a filamentary carrier, or a filament covered with a waterswellable non-powdery material, e.g. a water swellable polymer emulsion.

The already reported U.S. Pat. No. 5,157,752 proposes that, if thediameter of the passageway should be greater than two or more waterswellable elongate elements should be included with the fibers asrequired.

U.S. Pat. No. 6,633,709 (in the name of Sumitomo Electric LightwareCorp.) relates to a cable comprising a plurality of stacked fiber opticribbons having a plurality of water blocking yarns extending generallyalong the length of the stack of fiber optic ribbons and positionedaround at least a portion of the circumference of the stack wherein theplurality of water blocking yarns possess water swellablecharacteristics. The stack of fiber optic ribbons and the plurality ofwater blocking yarns extending along the length of the stack of fiberoptic ribbons are all loosely disposed in a buffer tube having aninterior channel larger than the stack of fiber optic ribbons. The swellcapacity of the plurality of water blocking yarns should exceed thecritical mass of water that could enter the buffer tube by a factor of2.0 or more. The swell capacity is determined as a function of thenumber of water blocking yarns, the yarn denier and the absorbency that,in turn, is given as a function of the yarn denier and expressed asswell mass per yarn mass. Thus, for a given number of yarns N, of denierd, and absorbency B the total capacity of water absorption expressed inmass per unit length. The critical mass of water is determined as afunction of the open area of the buffer tube and the water density.

The Applicant observes that neither the diameter of the retainingelement, nor its arrangement in term of number of fibers housed thereinis considered in this document.

SUMMARY OF THE INVENTION

The Applicant has noticed that the water swellable yarn should,preferably, not only offer an adequate water swelling capacity forpreventing the water propagation inside the micromodule, but also shouldshow a number of physical features of not minor importance for the goodmanufacturing and operation of the cable, further to be dimensionallycompatible with the retaining element size and the number of opticalfibers housed therein.

More particularly, the water swellable yarn should, preferably:

a) show a surface as smooth as to avoid friction against the opticalfiber, which can give rise to microbending; typically, frictions canoccur during the cable manufacturing process, installation and life;

b) have a thermal dimensional stability throughout the operating thermalrange of the cable so as not to cause stresses to the optical fibers;

c) show mechanical properties suitable with the manufacturing process ofthe optical cable, in particular ultimate tensile strength;

d) have an effective water absorption in term of both swelling volumeand rate of swelling reaction.

The Applicant found that among the above mentioned properties, thedimensional sizing and the swelling characteristics are particularlyimportant for preventing water propagation along the micromodule to suchan extent to make the micromodule fulfill with the requirement of theinternational standard.

Within the present invention, the Applicant perceived that when theretaining element has a reduced internal volume, in particular in caseof micromodules, i.e. when the retaining element is closely packed withoptical fibers and water swellable yarn, the swelling capacity in termof volume increase of the water swellable yarn is hindered and, as aconsequence, the ability of preventing the water propagation along themicromodule is impaired.

The Applicant found that the water propagation in an optical cable andin a micromodule containing optical fibers can be controlled below acritical value, without introducing microbending effects, by using awater swellable yarn arranged together and in contact with said opticalfibers. In particular the water swellable yarn is characterized by aswelling volume in a predetermined relationship with the free volume perfiber within the micromodule.

In other words, the Applicant found that the ability of preventing waterpropagation depends not only on the relationship of the swelling volumewith the free volume within the retaining element, but also on therelationship with the number of transmission elements housed within theretaining element.

According to another aspect, the Applicant found that the presence ofmechanical stresses during the manufacturing of the cable may causedifficulties.

Stresses can be generated during the step of bundling together thetransmission elements and the water swellable yarn in the manufacturingof the cable. In particular, the presence of a significant differencebetween the traction resistance of the water swellable yarn and that ofthe transmission elements may bring to ruptures or damages of one ofthem, or difficulties and irregularities of bundling.

In addition, since the retaining element is typically produced byextrusion, there is the possibility of adhesion of transmission elementsand water swellable yarn to the inner wall of the retaining element,particularly before the cooling of the latter has been completed.

Such adhesion may limit the freedom of movement of the transmissionelements during both the manufacturing and use of the cable, for examplein connection with cable laying, thermal excursions and the like.

The Applicant found that a powdery anti-friction agent enables toprevent such stresses, without requiring the use of fluid lubricants.

In particular, talc has been found suitable for providing the desiredanti-friction effect without causing microbending phenomena.

In a first aspect, the present invention relates to an optical cable forcommunication comprising:

-   -   a retaining element;    -   at least two transmission elements housed within said retaining        element; and    -   a water swellable yarn housed within said retaining element;        wherein the water swellable yarn is selected according to the        following equation:

$\begin{matrix}{\frac{V_{w}}{V_{TF}} = {\frac{k}{V_{t}} + R}} & (1)\end{matrix}$in which

-   -   V_(w) is the volume of the water swellable yarn after swelling        upon contact with water;    -   V_(TF) is the total free volume in the retaining element;    -   k is a constant ≧180    -   R is a constant ≧1.4; and    -   V_(t) is the free volume per each transmission element.

For the purpose of the present description and of the claims whichfollow, except where otherwise indicated, all numbers expressingamounts, quantities, percentages, and so forth, are to be understood asbeing modified in all instances by the term “about”. Also, all rangesinclude any combination of the maximum and minimum points disclosed andinclude any intermediate ranges therein, which may or may not bespecifically enumerated herein.

According to the present description and claims, the volumes of thetransmission elements, of the water swellable yarn and of the retainingelement of the invention are intended as volume per length unit, e.g.mm³/m and are calculated on the basis of the area of theircross-section. In the case of the retaining element, the inner volumeV_(i) is calculated on the basis of the inner area of the cross-section.

In one embodiment of the present invention, the retaining element has athickness of from 0.3 to 0.8 mm, and is hereinbelow indicated as “loosetube”. In this type of retaining element, the transmission elements maybe provided in bundles, in ribbons or in both of such configuration.

In another embodiment of the present invention, the retaining elementhas a thickness of 0.2 mm or less, for example 0.15 mm, and ishereinbelow indicated as “micromodule”. Within each micromodule, thetransmission elements may be arranged with or without clearance. As“clearance” it is herein intended, a difference between the innerdiameter of the retaining element and the diameter of the smallestcircle enveloping the transmission elements equal to or greater than 1%.

If no clearance is left between the optical fibers and the retainingelement, the micromodule is called “tight”, while a micromodule iscalled “of the loose type” when said clearance is present. At clearancevalue ≧1%, preferably up to 30%, for a suitable length of a micromodule(e.g. 1 m) it is possible to extract a single optical fiberindependently of the others.

Preferred according to the present invention is a micromodule of theloose type.

The total free volume V_(TF) is the volume inside the retaining elementleft vacant after the insertion of the transmission elements. It isdefined according to the following relationship:V _(TF) =└Vi−(V _(f) ×m)┘  (2)wherein

-   -   m is the number of transmission elements;    -   V_(i) is the inner volume of the retaining element; and    -   V_(f) is the volume of a single transmission element.

Advantageously, the retaining element is made of a polymeric material.

Suitable materials, according the specific needs, include: α-olefinpolymers and copolymers, such as low density polyethylene (LDPE), highdensity polyethylene (HDPE), linear low density polyethylene (LLDPE),ultra low density polyethylene (ULDPE); polypropylene; high and lowdensity poly-1-butene; poly-4-methyl-1-pentene; ultra;poly-4-methyl-1-pentene; ethylene propylene copolymers;ethylene-propylene-diene copolymers (EPDM); ethylene-1-butylenecopolymer, ethylene-vinyl acrylate copolymer, ethylene-methyl acrylatecopolymer, ethylene-butyl acrylate copolymer, ethylene-ethyl acetatecopolymer, ethylene-vinyl acetate copolymer,propylene-4-methyl-1-pentene copolymer, ethylene-vinyl alcoholcopolymer; ethylene-methyl acrylate-acrylic acid terpolymers; ormixtures thereof. Halogenated olefins, polymers and copolymers, may alsobe used, when absence of halogens is not required. Ethylene-butylacrylate copolymer, linear low density polyethylene (LLDPE), or mixturesthereof, are preferred.

Advantageously, an inorganic filler (b) is added to the polymericmaterial. The inorganic filler can include, for example, magnesiumhydroxide, aluminum hydroxide, aluminum oxide, kaolin, aluminatrihydrate, magnesium carbonate hydrate, magnesium carbonate, magnesiumcalcium carbonate hydrate, magnesium calcium carbonate, or mixturesthereof. Magnesium hydroxide, aluminum hydroxide, alumina trihydrate(Al₂O₃.3H₂O), or mixtures thereof, are particularly preferred.

Other additives, such as processing coadjuvants, lubricants, pigments,other fillers, may advantageously be added to the polymeric material.

As inner volume V_(i) of the retaining element it is herein intended thevolume per unit length confined within the retaining element.Preferably, the inner volume V_(i) is calculated on an inner diameter offrom 1 mm to 1.2 mm. Preferably, the retailing element has an outerdiameter of form 1.3 nm to 1.5 mm.

As V_(f) it is herein intended the volume per unit length of onetransmission element. Typically, in the case of optical fibers astransmission elements, their individual diameter of about 0.25 mm.Preferably the number of transmission elements is from 4 to 12.

The transmission elements can be arranged substantially parallel or,preferably, according to an open helix pattern (or SZ stranding) aroundthe longitudinal axis of the micromodule, i.e. the transmission elementsare stranded around said axis in sections with a first strandingdirection (S-shaped) alternating with sections with an oppositestranding direction of (Z-shaped).

The free volume V_(t), hereinafter also referred to as “free volume perfiber” is defined according the following:

$\begin{matrix}{V_{t} = {\frac{V_{TF}}{m} = \frac{\left\lfloor {V_{i} - \left( {V_{f} \times m} \right)} \right\rfloor}{m}}} & (3)\end{matrix}$

Advantageously, the water swellable yarn has a swelling time equal to orless then 2 minutes, as swelling time being intended the time forreaching at least 90% of the maximum expansion upon contact with water.

Examples of water swellable yarn useful for the present invention arepolyacrylate filaments or fibers optionally associated to polyesterfilaments or threads, and aromatic polyamide filaments or threads coatedwith a super-absorbent polymer, such as a polyacrylate.

According to the invention, the volume V_(w) of the water swellable yarn(hereinafter also referred to as “swelling volume”) is selectedaccording to the equation (1). The selection of V_(w) is correlated tothe number of transmission elements intended to be housed in theretaining element, and to the free volume per fiber V_(i) of theretaining element, as apparent by substituting V_(TF) in equation (1)with V_(t)·m according to equation (3) so to have:

$\begin{matrix}{\frac{V_{w}}{V_{t} \times m} = {\frac{k}{V_{t}} + R}} & (4)\end{matrix}$

By multiplying (4) for V_(t)·m, it is obtained:V _(w)=(k×m)+(R×V _(t) ×m)  (5)

According to another aspect, the present invention relates to a processfor manufacturing an optical cable comprising a retaining elementhousing at least two transmission elements and a water swellable yarn,said process comprising the steps of:

-   -   associating the transmission elements and the water swellable        yarn together to form a bundle;    -   extruding the retaining element around said bundle;        wherein the step of associating the transmission elements and        the water swellable yarn together comprises the step of applying        a powdery anti-friction agent over the transmission elements.

In the present description and claims, as “anti-friction agent” is meantan agent capable of reducing the friction and/or preventing the stickingamong the bundle components, i.e. transmission elements and waterswellable yarn, and of the bundle components to the retaining element.

According to the present process, the application of an anti-frictionagent in powdery form avoids the problems already mentioned above inconnection with water blocking greases or gels. Conveniently, thepowdery anti-friction agent should fulfill the specification of beingnon-hygroscopic and non-nutritive to fungus.

Preferably, in the process of the invention the step of associating thetransmission elements and the water swellable yarn together comprisesthe step of stranding the transmission elements and the water swellableyarn. Advantageously, said stranding step is an SZ stranding step.

Advantageously, the stranding step is effected after the step ofapplying a powdery anti-friction agent.

Advantageously, the step of applying a powdery anti-friction agentcomprises the step of advancing together the transmission elementsthrough a powdery anti-friction agent applicator.

Advantageously, the step of applying a powdery anti-friction agentcomprises the step of shielding the water swellable yarn from powderyanti-function agent application. For example, the shielding step can beeffected by advancing the water swellable yarn through a shielding tubepositioned inside the powdery anti-friction agent applicator. Theshielding step is preferred when the application of a powderyanti-function agent on the water swellable yarn could impair the properconformation of the optical unit because the water swellable yarn, inview of the features thereof, could drag an excessive amount of powder.

Advantageously, the powdery anti-friction agent applicator is providedwith a pneumatic wiping device. Said wiping device can be part of eitherthe applicator or a separate apparatus, arranged downstream theapplicator. The wiping device is useful for eliminating any surplusamount of powdery anti-friction agent from the surfaces of thetransmission elements and/or of the water swellable yarn.

Preferably, the powdery anti-friction agent is talc. Talc isadvantageous from the industrial point of view as nontoxic andeconomical.

Advantageously, the powdery anti-function agent has a grain sizesuitable for avoiding microbending phenomena. Preferably, the powderyanti-friction agent has an average grain size diameter D₅₀≦5 μm.

Preferably, the transmission elements are optical fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further illustrated hereinafter with reference tothe following examples and figures, wherein:

FIG. 1 shows a micromodule of the loose type according to the presentinvention;

FIG. 2 schematically shows an optical cable according to the inventioncontaining micromodules of the loose type;

FIG. 3 schematically illustrates an apparatus for performing the processaccording to the present invention;

FIGS. 4 and 5 illustrates water propagation test results;

FIG. 6 illustrates a plot of the relationship according the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts a micromodule 1 of the loose type according to anembodiment of the invention. The retaining element 2 has an outerdiameter of 1.46 mm, an inner diameter of 1.23 mm and a thickness of0.115 mm. The retaining element 2 encloses twelve transmission elements3 in form of optical fibers having a diameter of 0.245 mm, and one waterswellable yarn 4 with a diameter of 0.5 mm. The water swellable yarn 4is arbitrarily shown at the center of the retaining element 2, but inthe practice it is free to move inside the retaining element as far aspermitted by the transmission elements housed therein.

Suitable thermoplastic polymeric materials for the retaining element,according the specific needs, include: α-olefin polymers and copolymers,such as low density polyethylene (LDPE), high density polyethylene(HDPE), linear low density polyethylene (LLDPE), ultra low densitypolyethylene (ULDPE); polypropylene; high and low density poly-1-butene;poly-4-methyl-1-pentene; ultra; poly-4-methyl-1-pentene; ethylenepropylene copolymers; ethylene-propylene-diene copolymers (EPDM);ethylene-1-butylene copolymer, ethylene-vinyl acrylate copolymer,ethylene-methyl acrylate copolymer, ethylene-butyl acrylate copolymer,ethylene-ethyl acetate copolymer, ethylene-vinyl acetate copolymer,propylene-4-methyl-1-pentene copolymer, ethylene-vinyl alcoholcopolymer; ethylene-methyl acrylate-acrylic acid terpolymers; ormixtures thereof. Halogenated olefins, polymers and copolymers, may alsobe used, when absence of halogens is not required. Ethylene-butylacrylate copolymer, linear low density polyethylene (LLDPE), or mixturesthereof, are preferred.

Examples of olefins that may be used according to the present inventionand are available commercially are the product known by the name ofLotryl® from Atofina, Flexirene® from Polimeri Europa.

Advantageously, an inorganic filler (b) is added to the polymericmaterial. The inorganic filler can include, for example: magnesiumhydroxide, aluminum hydroxide, aluminum oxide, kaolin, aluminatrihydrate, magnesium carbonate hydrate, magnesium carbonate, magnesiumcalcium carbonate hydrate, magnesium calcium carbonate, or mixturesthereof. Magnesium hydroxide, aluminum hydroxide, alumina trihydrate(Al₂O₃.3H₂O), or mixtures thereof, are particularly preferred.

Examples of inorganic fillers which may be used for the retainingelement of the invention and are available commercially are the productsknown by the name of Hydrofy® from Sima or Atomfor® from Omya.

Other additives, such as processing coadjuvants, lubricants, pigments,other fillers, may advantageously be added to the polymeric material.

FIG. 2 schematically illustrates an optical cable 100 containing twelvemicromodules of the loose type 101, housed in a protecting tube 102 of athermoplastic polymeric material, such as the one known in the art asLS0H (Low Smoke Zero Halogen) or of medium or high density polyethylene(MDPE or HDPE), optionally added with a mineral charge such as magnesiumor aluminum hydroxide, and having an inner diameter of 6.4 mm and anouter diameter of 8.4 mm.

A longitudinal tape 103 is applied in radially external position overthe protecting tube 102, and separates the latter from sheath 106.Sheath 106 can be of MDPE or HDPE, optionally added with mineral charge,or of a LS0H material. In the present instance the sheath thickness isof 2.30 mm.

The longitudinal tape 103 eases the stripping-off of sheath 106 from theprotecting tube 102 during the cable termination. Two ripcords 104 areprovided in contact with the longitudinal tape 103, embedded in thesheath 106, and in diametrically opposite position each other.

Longitudinal reinforcements 105 are embedded in sheath 106, paralleleach other. Said longitudinal reinforcements 105 restrain longitudinalalterations of the cable due to thermo-mechanical stresses. Preferably,the longitudinal reinforcements 105 are tangentially positioned withrespect to the circumference of the inner diameter of sheath 106 so asto minimize the cable dimension. In the present instance, the diameterof the longitudinal reinforcement 105 is of 1.6 mm. The material ofthese components can be selected, e.g., from glass fiber reinforcedplastic, aramid/resin (aramid: aromatic polyamide) or steel.

The process for the manufacturing the cable is described in thefollowing with reference to the manufacturing apparatus 200,schematically depicted in FIG. 3. Pay-off standings 201 a,201 b areprovided for unwinding, respectively, a water swellable yarn and anumber of transmission elements, four in the present case. Waterswellable yarn and the transmissive elements are conveyed towards a talcapplicator 202 provided with inlet and outlet stationary distributorplates 203,204, for guiding and maintaining reciprocal positioning amongthe transmission elements and the water swellable yarn. The talcapplicator 202 is advantageously provided in downstream position with apneumatic wiping device 210 suitable for eliminating any surplus amountof talc from the surfaces of the fibers and of the water swellable yarn.

The tensile load applied to the optical fibers is usually in the rangeof from 50 to 100 g, and the tensile load applied to the water swellableyarn is typically of the same order of magnitude.

The presence of the talc enables a relative movement to take place amongthe optic unit components, i.e. fibers, water swellable yarn andretaining elements, avoiding or at least reducing the possibility thatunacceptable mechanical stresses are transmitted among the optic unitcomponents as a result of different payoff back tensions or of differentelongation/contraction loads.

Talc has been found particularly suitable, particularly because nosignificant microbending effect is detected in connection with its usein the resulting cable. Absence of significant microbending effects hasbeen found privileged with an average powder grain size diameter D₅₀≦5μm.

D₅₀ means that the 50% of the material passed a sieve of a predeterminedsize (5 μm in the case).

After the application of the powdery anti-friction agent, thetransmission elements and the water swellable yarn are conveyed to adistributor plate 205, preferably motorized, where they are stranded inan SZ stranding lay. For example, for twelve transmission elements andone water swellable yarn, the distributor plate 205 can provide ageometrical positioning 1+6+6.

The bundle resulting from the distributor plate 205 enters in anextruder 206 where it is inserted into a retaining element to provide,e.g., a micromodule. Said extruder 206 comprises an extrusion head, thedistance between the stranding device and the extrusion head beingcomprised between 280 mm and 700 mm. The micromodule is then made topass through a cooling through 207, that comprises water at atemperature, advantageously, of 20° C. The micromodule is then driven toa take-up system 209 by a line-pulling capstan 208.

Example 1 Optical Cables

Three optical cables with the design of FIG. 2 and an external diameterof 13.8 mm were manufactured with water-blocked micromodules, accordingto the following specifications:

-   -   protecting tube of HDPE internally buffered with water swellable        powder and talc; the protecting tube had an outer diameter of        6.1 mm and an inner diameter of 4.6 mm;    -   reinforcing armor provided in radially external position over        the sheath, and made of glass fibers, containing filaments        treated with water swellable powder;    -   water swellable longitudinal tape;    -   two ripcords;    -   two longitudinal reinforcements in glass fiber reinforced        plastic having a diameter of 1.7 mm;    -   thermoplastic sheath enveloping the longitudinal reinforcements,        being made of HDPE with a thickness of about 2.4 mm;    -   four micromodules SZ stranded with an oscillation angle of ±280°        and a pitch of 2 m, each comprising:        -   twelve optical fibers Pirelli NEON® each having a diameter            of about 0.245 mm;        -   a retaining element of LS0H material, based on LLDPE and EVA            as thermoplastic polymeric materials and magnesium hydroxide            as inorganic filler, having diameters and inner volume            according to Table 3; and        -   one water swellable yarn as from the following Table 1.

TABLE 1 V_(W) Swelling after 2 Cable Water swellable yarn [mm³/m]minutes (%) 1 GTB 150 3023 100 (667 dTex) 2 GTB 200 1915 100 (500 dTex)3 Twaron ® 1052 1897 88 (1750 dTex)

Swelling volume V_(w) and swelling percentage in time (swelling speed)were evaluated by means of a cylindrical container (diameter=75 mm)housing a piston with known weight (60 g), the latter being free tovertically move. A known length of the water swellable yarn to test (drysample) was positioned between the piston and the bottom of thecylindrical container, to compose a monolayered warp. The yarn was leftto swell in contact with bidistilled water flowing through holes in thepiston base. A micrometric comparator measured the movement of thepiston during the time.

The water swellable yarns were also tested for their mechanicalcharacteristics. The results are set forth in Table 2.

TABLE 2 Ultimate tensile Elongation at Cable Water swellable yarnstrength (N) break (%) 1 GTB 150 7 11 2 GTB 200 6 11 3 Twaron ® 1052 3502.7

GTB 150-667 dTex and GTB 200-500 dTex are water swellable yarns composedby polyacrylate swellable fibers on polyester support fibers(Geca-Tapes). Twaron® 1052-1750dTex is a water swellable yarnimpregnated with a super-absorbing polymer (Twaron Products V.o.F., TheNetherlands).

Example 2 Water Propagation Tests Along Water-Blocked Micromodules

The water propagation tests were performed according to the method F5Bprovided by international standard IEC 60794-1-2 (2001). In particular,the resistance to water propagation along the micromodules was evaluatedby applying 1 m water head for 24 hours at an end of micromodule sampleshaving length from 1 to 4 m. The cable samples contain micromodules asidentified in Example 1, but with different number of optical fibers, asspecified in the test results and comments.

It was noticed that micromodule samples with the same features in termof water swellable yarn and number of optical fibers, but with differentlengths did not provide significantly different test results.

Tests were effected on micromodules containing one water swellable yarnand a number of optical fibers ranging from 1 to 12, as well as thewater swellable yarn only. The inner volume V_(i) of each testedretaining element remained unchanged while varying the number oftransmission elements housed therein and, accordingly, the free volumeper fiber V_(t).

FIGS. 4 and 5 show the results of the series of tests performed oncables according to Example 1. For each cable type, identified by boththe water swellable yarn type and the number of fibers housed in themicromodule, No. 8 micromodule samples have been tested and the reportedvalues are the average of the single test results.

The water blocking effect performed by each water swellable yarn testedwas satisfactory in the absence of optical fibers in the micromodule.Increasing the optical fiber number housed in the micromodule, the waterpropagation lengthened. This is surprising because contrary to thehypothesis that the water propagation should be reduced by progressivelydecreasing the clearance (the total free volume V_(TF) and the freevolume per fiber V_(t)) left to the water longitudinal propagation.

In particular, FIG. 4 shows the influence of the transmission elementnumber over the water blocking capacity of the water swellable yarn. Byincreasing the number of transmission elements, the water propagationalong each micromodule increased. In cable 1 containing the GTB 150 aswater swellable yarn, the water propagation was confined in about 1 meven when the transmission elements amounted to 12; in the other cables,containing water swellable yarn having a V_(w) remarkably smaller thanthat of GTB150, such a limited water penetration was observed with anumber of transmission elements up to 3-4.

Tests for cable 1 went on for 14 days (test suspended at the 15^(th)day) and the water propagation never reached the 200 cm length. Morespecifically, cable 1 samples containing 12 optical fibers showed topwater propagation length of 68-145 cm after 14 day-test.

FIG. 5 show that the water propagation along the tested micromoduleschanges as a function of the free volume per fiber V_(t). The additionof optical fibers progressively reduced V_(t). The swelling volume ofthe water swellable yarn is one of the factors limiting the waterpropagation. The chart of FIG. 5 attests that water swellable yarns withlower V_(w) can efficiently perform only for free volume pertransmission element V_(t) larger than the predetermined value.

The following Table 3 summarizes the geometrical characteristics of themicromodule components, the volume relationship described by theequation of the invention, and the results obtained from the waterpropagation tests.

TABLE 3 Water swellable Water propaga- Retaining yarn tion lengthelement V_(i) V_(w) V_(f)xm V_(t) V_(TF) (average) ID OD/ID [mm] [mm³/m][mm³/m] m [mm³/m] [mm³/m] [mm³/m] V_(w)/V_(TF) [m] FIG. 6 1.46/1.211149.9 GTB150 12 565.7 48.68 584.2 5.17 1.02 A 3023 10 471.4 67.85 678.54.45 0.84 L 8 377.1 96.59 772.8 3.91 0.69 M 6 282.6 144.51 867.0 3.490.59 N 1.46/1.18 1093.59 GTB200 12 565.7 43.99 527.9 3.63 4.19 B 1915 10471.4 62.22 622.2 3.06 3.04 G 8 377.1 89.55 716.4 2.67 2.12 H 6 282.6135.12 810.7 2.36 1.44 I 4 188.6 226.25 905.0 2.12 0.99 C 1.46/1.181093.59 Twaron ® 12 565.7 43.99 547.9 3.56 4.75 D 1897 3 141.4 317.39952.2 1.99 0.96 E 1.46/1.23 1188.23 GTB150 12 565.7 51.88 622.5 4.861.02 F 3023 OD = outer diameter ID = inner diameter The Twaron ® yarn isa type as from previous Table 1 and 2. The water propagation dataunderline are those according to the F5B of international standard IEC60791-1-2 (2001).

FIG. 6 illustrates the relationship V_(w)/V_(TF) as a function of thefree volume per fiber V_(t) which, in turn, depends on the number oftransmission elements housed in the retaining element. The curve in FIG.6 is for equation (1) of the invention wherein k=182 and R=1.42. Some ofthe points of FIG. 6 correspond to the experiments detailed in Table 3and are identified by the ID letter. The points D, B, G, H, and I, belowthe curve, correspond to experiments resulting in a water migrationappreciably longer than 1 m. The points L, M, and N, above the curve,correspond to experiments resulting in a water migration shorter than 1m.

In particular, for high V_(t) values, typical of loose tube design,V_(w)/V_(TF) is nearly constant, whereas for low V_(t) values, typicalof the micromodule construction, V_(w)/V_(TF) is strongly affected bythe variation of V_(t). At constant V_(TF), the behavior issignificantly different in case of few transmission elements or manytransmission elements housed in the retaining element.

1. An optical cable for communication comprising: a retaining element;at least two individual optical fibers housed within said retainingelement; and a water swellable yarn housed within said retaining elementand arranged between and in contact with said at least two individualoptical fibers, wherein the water swellable yarn is selected accordingto the following equation: $\begin{matrix}{\frac{V_{w}}{V_{TF}} = {\frac{k}{V_{t}} + R}} & (1)\end{matrix}$ in which V_(w) is the volume of the water swellable yarnafter swelling upon contact with water; V_(TF) is the total free volumein the retaining element; k is a constant ≧180 mm³/m; R is a constant≧1.4; and V_(t) is the free volume per each transmission element.
 2. Theoptical cable according to claim 1, wherein the retaining element has athickness of 0.3 to 0.8 mm.
 3. The optical cable according to claim 1,wherein the retaining element has a thickness equal to or less than 0.2mm.
 4. The optical cable according to claim 3, wherein the retainingelement has a clearance equal to or greater than 1%, with respect to theoptical fibers, prior to any swelling of the swellable yarn.
 5. Theoptical cable according to claim 1, wherein the retaining element has aninner diameter of 1 mm to 1.2 mm.
 6. The optical cable according toclaim 1, wherein the transmission elements are optical fibers.
 7. Theoptical cable according to claim 1, wherein there are 4-12 transmissionelements.
 8. The optical cable according to claim 1, wherein thetransmission elements are SZ stranded.
 9. The optical cable according toclaim 1, wherein the water swellable yarn has a swelling time equal toor less than 2 minutes.
 10. An optical cable according to claim 1,wherein the water swellable yarn is selected from polyacrylate filamentsor fibers, optionally associated with polyester filaments or threads,and aromatic polyamide filaments or threads coated with asuper-absorbent polymer.